An electrophotographic image forming apparatus, such as laser printers, includes a thermal fixing device. Typically, the fixing device includes a heat roller and a pressure roller for nipping a sheet of paper therebetween to thermally fix a toner-image deposited on the sheet. The fixing device further includes a temperature-sensing member, such as thermistors, for sensing temperature of the heat roller and a control unit controls the temperature thereof to fall within a predetermined range.
The fixing device further includes a thermostat for preventing the heat roller from being overheated. Overheat of the heat roller may be caused by runaway of the control unit. If the thermostat is actuated to interrupt the electrical connection, a heating source for heating the heat roller is disconnected from a power supply so that overheat of the heat roller can be prevented.